1. Field of Invention
This invention generally relates to the optical inspection of electronic parts and more specifically to an autofocus system for use in such optical inspection.
2. Description of Related Art
Optical inspection of electronic parts such as such as wafers, circuit boards, flat panel displays, multi chip modules, and high-density electronic packages requires the use of high resolution optics to detect small defects in the part. In addition the high resolution optics part must be kept in focus during the entire inspection to see the defects. FIG. 1 is the example of a part 1-1 with variations in surface height Z in both the scanning direction 1-2 and in an orthogonal direction relative to the scanning direction. Newly introduced circuit boards with embedded dies are examples of such parts and are recognized as being difficult to image because the surfaces being imaged are subject to height variations caused by the embedded die or circuits placed on and between the substrate layers and the recognized characteristics that the substrate itself will warp.
FIG. 1 is helpful in understanding problems that exist when prior art apparatus is required to image defects in various parts. In the prior art, a camera, not shown, scans over the surface of a part in a scanning direction 1-2. As the scan images an area such as shown in FIG. 1 which is transverse to the scanning direction 1-2, the image taken by the camera must be in focus. The part shown in FIG. 1 has a range of height variations, shown by arrows 1-3 that must be within the depth of field of the camera optics. With prior art imaging apparatus, a particular focus point selected for the camera could arbitrarily be at the top 1-5 of or the bottom 1-6 of the part or at any intermediate position. Given this, optics design of the imaging optics sets the required depth of field, preferably twice the distance between the top 1-5 and bottom 1-6 of the part as shown by arrows 1-7 and 1-8 that depict a depth that will cover the range of height variations. However, as known and described in greater detail later, the depth of field for an optics system also determines the resolution of the image. Such resolutions often limit image quality that will prevent the detection of small defects in the part.
To inspect the part for defects a camera is frequently used to scan the part in a serpentine pattern as illustrated by the contiguous strips A through E in FIG. 2. The width of the camera's field of view is represented by rectangle 2-1. Various techniques have been described in the art to maintain focus during such inspections. U.S. Pat. No. 7,015,445 to Bishop for “Method for Optimizing Inspection Speed in Low, and Fluorescent Light Applications without Sacrificing Signal to Noise Ratio, Resolution, or Focus Quality” describes the use of a triangulation sensor to maintain a constant distance between the imaging optics and part as the part is scanned. FIG. 3 shows a wavy part 3-1 at an incline θ, imaging optics 3-2 and an imaging camera 3-3. As the part is scanned the imaging optics 3-2 and imaging camera 3-3 are raised and lowered as a unit to keep the surface of the part within the optical depth of field 3-4 of the imaging optics. Conceptually the part, the optics, or the combination of the optics and camera can be moved to maintain focus.
FIG. 4 shows the use of a triangulation sensor with an optical source 4-1, illumination beam 4-2 and position sensor 4-3. The triangulation sensor scans ahead of the camera as indicated by arrow 4-4. The position at which optical beam 4-5 hits the position sensor indicates the distance to the part 4-6. In this FIG. 4, the imaging camera optics has a depth of field (DOF) 4-7. This distance measurement is used in a feedback loop to mechanically move either the imaging optics or the part relative to each other to maintain focus. FIG. 5 shows how the position of the beam onto position sensor 5-1 moves as a function of the distance to the part. Three surfaces at different distances represented by surfaces 5-3, 5-4, and 5-5, are projected onto sensor 5-1 at positions 5-3′, 5-4′, and 5-5′ respectively. These distance measurements are used in a feedback loop mechanically to move either the optical head or the part as a function of the measured height to maintain focus.
There are two limitations to these focusing methods. First, if an illumination beam 4-2 in FIG. 4 hits the part at a material boundary the distance measurement may be incorrect. Referring to FIG. 6 and specifically to FIG. 6A, when optical beam 6-1 from the triangulation sensor hits highly reflective material 6-2, the entire illumination spot 6-3 sits on material 6-2. The image of this spot creates a symmetrical shaped beam 6-4 on sensor 6-5. If the material beneath the sensor now changes, as the part is scanned to one with a lower reflectivity, represented by 6-6 in FIG. 6B, a lower intensity spatially symmetrical spot represented by 6-7 is projected onto sensor 6-5. So long as projected spots 6-7 and 6-4 are spatially symmetrical, the center of mass of the spots, which represents the distance to the part, will be the same and the correct focus distance will be calculated. If, however, the illumination spot 6-3 falls on a material boundary as in FIG. 6C, it is spread between highly reflective material 6-2 and lower reflective material 6-3. In this event, the spot projected onto the sensor will not be symmetrical and the distance to the part will be incorrectly calculated to be 6-8 when the correct distance should be 6-7 because the center of mass of the spot no longer represents the correct distance to the part
Second, in FIG. 7 an imaging camera 7-1 with imaging optics 7-2 moves in the focus direction along a Z axis 7-3 to maintain constant distance to the surface of part 7-4 while focus distance is adjusted dynamically as the part is scanned in the Y direction. Focus distance for the entire imaging camera is based on a series of single point measurements along a narrow line in the direction of scan. No measurements are taken perpendicular to the direction of scan. This implies that across the width of the camera, or width of each scanned strip A-E shown in FIG. 2 all features on the surface must lie within the optical depth of field of the imaging optics indicated by arrow 3-4 in FIG. 3. As will be apparent any feature not within the depth of field will be out of focus.
As the part is scanned, the focus measurement unit may pass over high or low features in the part. Some focus distances may be calculated based on the distance to a high feature while other focus distances may be calculated based on the distance to a low feature. This implies that the optical depth of field of the imaging optics must be sufficiently large to insure proper focus regardless of whether a high or low feature was beneath the focus measurement unit at the time when the focus measurement was calculated. Calculating focus based only on measurement values along a line in the direction of scan will have this limitation, regardless of. how many measurements are acquired, how fast the calculations are computed, the specific method of measurement or type of measurement device. A preferred device is a single point triangulation sensor; single-point confocal sensors, single point capacitive sensors and others may be substituted depending upon the performance criteria to be provided by the inspection apparatus.
For current focus tracking technology to properly function the depth of focus of the imaging optics, indicated as arrow 8-1 in FIG. 8, must be sufficiently large to guarantee focus for all the possible features heights that may be used to calculate the focus distance. It is important to note that FIG. 8 represents the surface of the part in the X axis which is perpendicular to the direction of mechanical scan. FIG. 8 also represents the image projected onto the long axis of a linear CCD scan camera as represented by block 2-1 in FIG. 2. Unfortunately requiring this large depth of field seriously limits the spatial resolution and defect detection capabilities of the inspection system. More specifically, the optical depth of focus (DOF) is given by the equation:
      D    ⁢                  ⁢    O    ⁢                  ⁢    F    =      λ          2      ⁢                          ⁢      N      ⁢                          ⁢              A        2                            and Resolution is given by the equation:        
  Resolution  =      λ          2      ⁢                          ⁢      N      ⁢                          ⁢      A                      where:                    λ=Wavelength of light imaged onto the camera, and            NA=numerical aperture of the imaging optics                        
As known and demonstrated by the foregoing relationships, large depth of focus (DOF) requires a small numerical aperture (NA) while high resolution requires a large numerical aperture. As (NA) becomes smaller, the level of light reaching the imaging camera, also decreases and this impacts the contrast in the final image. These criteria impose limitations on the inspection of parts that can prevent the construction of imaging optics with both a large depth of focus and a high resolution. As will be apparent, if the part being inspected must stay in focus, current inspection systems sacrifice the resolution of the imaging optics which thereby inherently limits the ability to detect small defects.
Table 1 is a list of commercially available lenses from the Zeiss Corporation. The table lists the depth of focus, numerical aperture, resolving power, light collection coefficient, light collection cone angle, working distance magnification and part number for each lens.
TABLE 1Commercially Available Objective Lenses From Zeiss      Magnification/ pixel size (microns)      Zeiss Part Number      Numerical Aperture (NA)    Light Collection Coefficient (NA2)  Light Collection Cone Angle (degrees) θ = 2xsin1(NA)Resolving Power for λ = 0.55 (Microns)   λ      2    ⁢    xNA  Depth of Focus λ = 0.55 (Microns)   λ      2    ⁢          xNA      2            Working Distance (WD)1.25x/10.44423000.0350.00124.07.82293.9mm2.5x/5.24423100.0750.00568.63.6499.4mm  5x/2.64403200.150.0225171.812.213.6mm  5x/2.6—0.250.0625291.14.4— 10X/1.34428320.250.0625291.14.412.7mm 10X/1.34423300.300.0900350.93.15.7mm  20X/0.654428400.400.1600470.71.79.8mm 10X/1.34401350.500.2500600.51.12.0mm  20X/0.504423400.500.2500600.51.11.4mm
Note that the 1.25× lens with an NA of 0.035 has a depth of focus of 229 microns whereas the 20× lens with an NA of 0.50 only has a depth of focus of 1.1 microns. Unfortunately, unless all the features in the field of view of the inspection camera vary in height less than 1.1 microns, the 20× 0.5 NA lens cannot be used to inspect the part. Therefore many inspection systems are forced to use low NA optics to maintain focus and are unable to inspect very small features that require high magnification and high resolution.